This invention relates to polishing materials particularly to polishing materials based on diamond that are used to produce fine finishes on hard materials.
Certain very hard ceramic materials are extremely difficult to polish to a fine finish unless a diamond polishing slurry is employed. Such materials include for example silicon carbide, aluminum/titanium carbide, tungsten carbide, aluminum nitride and alpha alumina monolith bodies. Diamond is very expensive and thus the polishing process itself is very costly.
Typical diamond slurries for this application contain diamond particles less than 2 microns in diameter and contain from about 10 to about 20 carats per liter of the diamond. The diamond particles need to be kept in suspension and this is typically done using suspending agents. U.S. Pat. No. 5,149,338 however teaches the use of a colloidal boehmite to maintain a diamond abrasive powder in suspension. This is said to yield a finish below 20 angstroms RMS when used on silicon carbide at a pH of 4. The boehmite has no significant abrasive qualities and is merely a means of presenting the diamond particles to the surface in a more efficient way. Moreover the pH used is severely corrosive of all but the best quality stainless steel, (or other resistant material), for the equipment contacted by the slurry. Other materials are available for polishing such hard ceramics including colloidal silica but while these are undoubtedly cheaper than diamond they are generally less effective and take much longer to achieve an acceptable finish.
There is therefore a need for a cheaper alternative to diamond for polishing hard materials that does not have a deleterious effect on the equipment used. This need is filled by the present invention which is more effective than diamond used alone and yet is quicker to achieve its desired surface finish. While not so cheap as colloidal silica it is substantially less expensive that a pure diamond polishing slurry.